TDSCDMA (Time Division Synchronous Code Division Multiple Access) is a wireless radio standard for the physical layer of a 3G (third generation) air interface. Different from WCDMA and CDMA2000, which adopt a frequency division duplex, TDSCDMA is designed for time division duplex/multiple access (TDD/TDMA) operation with synchronous CDMA technology.
TDSCDMA uses time domain duplexing in combination with multiple access techniques to support both symmetrical and asymmetrical traffic. The variable allocation of time slots for uplink or downlink traffic allows TDSCDMA to meet asymmetric traffic requirements and to support a variety of users. In TDSCDMA systems, multiple access techniques employ both unique codes and time signatures to separate the users in a given cell. The TDSCDMA standard defines a frame structure with three layers: the radio frame, the subframe and the time slot. The radio frame is 10 ms. The subframe is 5 ms. and is divided into seven time slots. A time slot has four parts: a midamble, two data fields on each side of the midamble and a guard period. The receiver uses the midamble to perform channel estimation.
In CDMA systems, many users access the same channel simultaneously. Each user is separated from the others by a code known as the spreading code. However, each new user added to the system produces interference with the other users. In CDMA systems, this multiple access interference (MAI) is the limiting factor in system capacity.
Multiple access interference equally affects all users in a CDMA system. To deal with this, other systems use detection schemes such as the rake receiver. However, rake receivers are suboptimal because they consider only the user's signal information in the detection process, with no attempt to characterize the interference from the other users. By contrast, joint detection algorithms process all users in parallel and thus include the interference information from the other users. Joint detection schemes are complex and computationally intensive. Complexity grows exponentially as the number of codes increases. Joint detection is well-suited to TDSCDMA systems because the number of users in a time slot is limited to 16. The result is a joint detector of reasonable complexity.
In traditional communication systems, the baseband receiver includes two main components: an inner receiver, also known as an equalizer or a chip rate processor, which mitigates the effects of multipath and interference, and an outer receiver which performs channel decoding and other symbol rate processing. Circuitry for implementing a TDSCDMA baseband processor may use different approaches, ranging from a programmable digital signal processor to application-specific integrated circuits (ASICs). The programmable digital signal processor has the advantage of flexibility for different applications but may not have sufficient computation speed to process TDSCDMA signals in real time. ASICs may have higher computation speed but have limited flexibility for different applications and different processing algorithms.
Accordingly, there is a need for TDSCDMA architectures and implementations which achieve high computation speed, flexibility and programmability.